I've played as a Tav Paladin. And I've also given Astarion a couple of levels of Paladin on one playthrough. There seems to be no reaction - you don't get made Oathbreaker with either route, and if its the Ascended route the Gur start the fight so killing them doesn't break your oath either
It feels like such a perfect spot for it though, given the gravity of the situation. Odd.
